media: staging/intel-ipu3: update minimal GDC envelope size to 4

The ipu3 GDC function need some envelope to do filtering and the
minimal envelope size(GDC in - out) for ipu3 should be 4.
Current value 4 was defined for older version GDC, this patch
correct it.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>